The present invention relates to ground vehicle speed measuring apparatus and more particularly to apparatus for removing spurious signal components from the output of a doppler radar ground vehicle speed sensing system.
Many types of ground vehicles employ speedometers coupled into the drive train of the vehicle for measuring their ground velocity. This speed measuring technique, however, presumes that there is a known and unchanging correlation between speed of the drive train and speed of the vehicle. Consequently, it will provide an accurate speed measurement only if the vehicle wheels neither slip nor substantially change in radius throughout their working life. The occurrence of either of these conditions will introduce unknown errors into the speed measurements. In railroad vehicles, for example, the diameter of the wheel may wear by as much as several inches throughout its operating life. The relationship between the rotational speed of the wheel axle and the ground speed of the vehicle will therefore also change throughout the life of the wheel. In other applications, such as farm tractors, all-terrain vehicles, etc., substantial wheel slip occurs such that again the rotational velocity of the drive train cannot be directly correlated to the ground velocity of the vehicle.
Doppler radar velocity sensors have been used in some applications to provide a more direct measurement of the speed of the ground vehicle. In these applications a radio frequency (RF) signal is transmitted towards, and reflected from, the ground over which the vehicle is passing. The shift in frequency of the reflected signal relative to the transmitted signal is indicative of the ground velocity of the vehicle. Patents disclosing systems employing this approach include Augustine, U.S. Pat. No. 3,833,906, Augustine et al., U.S. Pat. No. 3,859,660, Clorfeine, U.S. Pat. No. 4,050,071 and Caplan, U.S. Pat. No. 4,107,680.
While the foregoing doppler velocity measurement technique is immune to wheel slip, wheel wear and associated problems, it nonetheless suffers from problems of its own. Doppler radar measurement systems which employ only a single doppler sensor rely upon the boresight of the receiving antenna associated with the doppler velocity sensor having a preestablished angular orientation with respect to the ground. Changing pitch of the vehicle causes this angular orientation to vary, however, thereby introducing errors into the velocity measurement. The aforementioned patents address this problem by providing two doppler velocity sensors, pointed in different directions, and circuitry for combining the outputs of the sensors so as to compensate to some extent for pitch of the vehicle.
Additional error components are introduced by the substantial and widely varying linear and angular accelerations which some types of vehicles encounter as they maneuver over broken ground. This is particularly true of off-the-road vehicles such as farm tractors which must regularly navigate quite rough and uneven terrain. The irregular linear and angular accelerations which the vehicle undergoes as it passes over such terrain introduces substantial spurious components into the doppler output which may in some circumstances completely mask the desired linear velocity measurement.
In aircraft navigational systems employing doppler radar to detect ground velocity, inertial platforms have in the past been provided for sensing pitch, roll and yaw of the aircraft and for then correcting the doppler output in accordance therewith. Patents relating to such systems include the patent to Parr et al., U.S. Pat. No. 3,028,592, Tholey et al., U.S. Pat. No. 3,462,586 and Gray, U.S. Pat. No. 3,810,176.
The inertial system described in the Parr et al. patent uses several accelerometers mounted on a gyroscopically stabilized platform to generate navigational data. The inertial system interacts with a doppler radar system in varying degrees, depending upon the operational mode of the apparatus.